Today most backup systems operate by having the network administrator identify a time of day during which little or no network activity occurs. During this time the network administrator turns the network over to a backup system and the data files stored on the computer network are backed up, file by file, to a long term storage medium, such as a tape or hard drive backup system. Computer backups are performed using several strategies. The simplest entails a complete transfer of all data and meta-data (such as time stamps, ownership, and access rights) to a target which is simple but redundantly transfers data already present on the target at potential high expense. Incremental backups transferring actual changes or a more manageable subset are also possible. Common mechanisms for determining an appropriate increment include archive bits and modification time stamps. Archive bits are set by the operating system on any change and reset by the backup software but preclude use for multiple backup systems and don't narrow down the types of change. Modification time stamps are set by the operating system but can sometimes be adjusted by user software.
A full backup of a large data set may take a long time to complete. To avoid downtime, high-availability systems may instead perform the backup on a snapshot—a read-only copy of the data set frozen at a point in time—and allow applications to continue writing to their data. Most snapshot implementations are efficient and can create snapshots in O(1). In other words, the time and I/O needed to create the snapshot does not increase with the size of the data set; by contrast, the time and I/O required for a direct backup is proportional to the size of the data set. In some systems once the initial snapshot is taken of a data set, subsequent snapshots copy the changed data only, and use a system of pointers to reference the initial snapshot. This method of pointer-based snapshots consumes less disk capacity than if the data set was repeatedly cloned. However, in today's high speed environment, each computer can create many overlapping snapshots with different job runs, which take up disk space and slow accesses down.